Nitric oxide (NO) is the proposed transcellular signal in parts of the cardiovascular system, the gastrointestinal (GI) tract, the central nervous system and as well as derivatives of transformed hematopoietic stem cells. Although NO is the leading candidate for endothelial derived relaxing factor (EDRF), it has recently been proposed to be the primary inhibitory neurotransmitter released by non-adrenergic non-cholinergic (NANC) nerves in the gut wall. In intact colon, exogenous NO treatment results in both a transient hyperpolarization and a mechanical relaxation. The NO response in colon is also associated with an increase in intracellular cGMP levels. Studies in other cell types have proposed cGMP as a regulator of both K+ and Ca2+ channels. The objective of this proposal is to investigate the conductances responsible for the NANC-mediated changes in membrane potential found in the circular smooth muscle layer of canine colon. Specifically, this study will utilize whole cell patch clamp to (1) identify the changes in ionic conductances caused by NO, (2) determine whether cGMP production mediates the changes in conductances elicited by NO, and (3) compare the pharmacology of NO and its effects on isolated cells with intact tissue data to evaluate NO as a NANC neurotransmitter. Elucidation of the NO- induced conductance changes and the second messenger pathways which may subserve these changes, will add to our understanding of inhibitory responses of the GI tract such as receptive relaxation and descending inhibition in response to distension. This knowledge will also aid in our understanding of signal transduction in other tissues where NO is a proposed intercellular mediator.